Embryology Flashcards
Where does the allantois originate from?
Evagination of the secondary yolksac (future hindgut) endoderm. Will stick out into the extraembryonic (somatopleuric) mesoderm alongside the umbilical arteries and umbilical vein, since the connecting stalk forms the vessels which communicate with the placenta.
What does the allantois give rise to in the fetus? What does it communicate with?
The distal part (away from the cloaca, communicating with the connecting stalk / umbilical cord ) should involute into a fibrous band.
The remainder of the allantois forms the urachus (which communicates with the umbilicus and also involutes into a fibrous band). the urogenital sinus, and the urethra.
It communicates with the hindgut structures of the urogenital sinus, which will form the bladder and the urethra.
What will patent urachus present with?
Communication between bladder and umbilicus via urachus -> urine leakage from the umbilicus
What give rise to the median and medial umbilical ligaments?
MediaN = allaNtois
MediaL = umbilicaL arteries (paired)
What causes congenital umbilical hernia?
Incomplete closure of umbilical ring, due to defect in linea alba.
Reducible, asymptomatic, benign, and self-resolving. Usually minimal inclusion of abdominal contents.
How do you tell a vitelline fistula apart from a patent urachus?
Vitelline fistula will show meconium draining thru the umbilical ring.
Remember, vitelline fistula is due to Meckel’s diverticulum persisting and communicating with umbilicus. It, along with the urachus, normally communicates with the umbilicus. However it is present due to the vitelline duct, which connects the MIDGUT to the yolksac.
What goes wrong in a PAX6 mutation?
6543
Eye development is controlled by the PAX6 gene.
Mutation of PAX6 gene -> congenital aniridia
What is the most important transcription factor in guiding the segmental organization of the embryo?
Hox (Homeobox) genes
-> if you aren’t sure of a transcription factor, guess this.
What area of the embryo releases Sonic Hedgehog? What does it do?
Zone of polarizing activity - controls development of anteroposterior axis of the limbs
Also controls CNS development -> deficiency leads to holoprosencephaly (as in Patau syndrome)
What two growth factors are released at the end of the limb, and what is this limb end called?
Apical epidermal ridge - releases FGF and Wnt-7
What do FGF and Wnt-7 control?
FGF - controls elongation on the proximal-distal axis
Wnt-7 - controls the only axis we haven’t mentioned so far -> dorsal-ventral
What is the morula?
The compacted embryo, in stages after the third cleavage (8-cell stage). Blastomeres (individual cells) adhere tightly to eachother and cell divisions between blastomeres are no longer visible (squished together)
When does the morula turn into a blastocyst?
Via cavitation -> formation of a cystic cavity, the blastocoel.
Cavity has trophoectoderm on the outside and an inner cell mass
When does implantation occur?
Between days 6-8, this is when the trophoectoderm differentiates into syncytiotrophoblasts and begins secreting hCG
What is the definition of gastrulation?
The formation of the trilaminar embryo
How is the extraembryonic coelom formed?
Small lacunae appear in the extraembryonic mesoderm (between Heuser’s membrane which lines the primary yolk sac, and the trophooblast layer) and coalesce. Within 24 hours, this chorionic cavity is formed.
What are the two layers of the extraembryonic mesoderm after the chorionic cavity is formed?
- Splanchnopleuric mesoderm - lines the primary yolk sac
2. Somatopleuric mesoderm - lines the cytotrophoblast and continues around amniotic cavity
What forms the secondary / definitive yolk sac?
Cavity lined by migrating extraembryonic endoderm, it is much smaller than primary yolk sac.
-> While the chorionic cavity is forming, a second wave of hypoblast cells proliferates and displaces the Heuser’s membrane lining the primary yolk sac. These hypoblast cells which line the splanchnopleuric mesoderm become the extraembryonic endoderm, forming the yolk sac. The cells which are displaced are the primary yolk sac, which turns into cysts and are displaced to the opposite side of the embryo.
What is the function of the connecting stalk?
It is a mass of extraembryonic mesoderm which connects the embryo to the chorionic plate, since the chorionic cavity is getting much larger.
What is the chorionic plate?
Also called chorion, it is the wall of the chorionic cavity. It is somatopleuric mesoderm + cytotrophoblast + syncytiotrophoblast
What is the role of the secondary (definitive) yolk sac in humans?
Transfer nutrients in weeks 2 and 3 before the uteroplacental circulation is established. Hemopoesis first occurs in the vascularized extraembryonic splanchnopleuric mesoderm until week 6 it starts in liver. Will also give rise to primordial germ cells in endodermal layer.
What are the three stages of transformation of the chorionic stem villi?
- primary - columns of cytotrophoblast surrounded by syncytiotrophobolast
- secondary - extraembyronic somatopleuric mesoderm invades to form core
- tertiary - mesenchymal cells in the extraembryonic somatopleuric mesoderm differentiate into capillaries and blood cells
What are the caudal and cranial ends where the ectoderm and endoderm adhere?
Cranial: Buccopharyngeal
Caudal: Cloacal membrane
What is the allantois?
Structure forming from outpouching of extraembryonic endoderm from caudal wall of definitive yolk sac into the connecting stalk. It will be incorporated into the umbilical cord later, and is rudimentary in humans